Roadways generally consist of several layers.
A top layer, which is in direct contact with road vehicles and which can be made from a conventional road asphalt, for example asphalt concrete (in the following, this layer shall be referred to as the "top layer"). PA1 At least one, and sometimes several, lower layers constituting the roadway foundation, and which can be made from materials treated with a hydraulic binder such as cement aggregate or slag aggregate or from a road asphalt (in the following, this layer shall be referred to as the "foundation layer" of the roadway). PA1 first, the deformation and tensile strains become localized at the base of the foundation layer, the maximum effect occurring at the bottom of the defects, PA1 then microcracks are formed appearing most typically at the bottom of the defects, PA1 and finally, when the strains exceed the tensile strength of the material, the microcracks are transformed into cracks propagating toward the top of the roadway and finally cause the roadway to rupture. PA1 creating an interface consisting of a thin asphalt/rubber layer which is poured onto the foundation layer to separate the movements of the latter from those of the surmounting top layer; PA1 enhancing the tensile strength of the surface layers of asphalt concrete with polyester mesh presenting adequate mechanical characteristics, said mesh being placed between the foundation layer and the top layer; PA1 installing a nonwoven polyester material and covering said nonwoven material with an approximately 5 cm thick top layer of an asphalt concrete; PA1 installing a nonwoven geotextile interface impregnated with an asphalt binder consisting of an asphalt modified with, for example, a copolymer of styrene and a conjugated diene such as butadiene, as described in French Patent Application FR-A-2 592 411 (or in equivalent U.S. Pat. No. 4,834,577, the content of this and of the other patent references cited herein being incorporated herein by reference); PA1 installing a layer of a geotextile impregnated with a first asphalt binder bonded to a layer of aggregate coated with a second asphalt binder, as described in International Patent Application WO-94/01623 (or in equivalent U.S. Pat. No. 5,445,473). PA1 a top layer, PA1 at least one rigid foundation layer with a high modulus of rigidity and supporting the top layer, PA1 the ground, or an untreated material, or a damaged roadway, supporting the foundation layer, said roadway structure being characterized in that it comprises, beneath the foundation layer whose modulus of rigidity is higher than or equal to 14.times.10.sup.3 MPa, a layer adhering to said foundation layer and whose thickness is such that it constitutes a support of smooth and flat surface for the foundation layer. PA1 the ground or an untreated material or a severely degraded roadway is covered with a layer intended to adhere to the foundation layer and whose thickness is such that it constitutes a support of smooth and flat surface for said foundation layer, PA1 to this layer is applied the foundation layer having a modulus of rigidity greater than 14.times.10.sup.3 MPa, PA1 the foundation layer is covered with the top layer.
In the process of building a roadway, these layers can be laid directly on the ground, or also on an untreated material (such as, for example, an untreated sand-gravel mixture or a reconstituted, moistened sand-gravel mixture).
In the process of strengthening a damaged roadway, the new layer or layers of treated materials are placed directly in contact with the top layer of the old roadway.
It is known that road specialists are often interested in using a foundation layer of great structural strength, namely one of high rigidity, which makes it possible to reduce layer thickness and, hence, roadway construction costs.
It is known that under the effect of tensile stresses caused by the road traffic, deformations and strains develop at the base of the layers, particularly at the base of the foundation layer. The repetition of these deformations and strains leads to mechanical fatigue which is responsible for cracking.
This well-known phenomenon occurs in several stages:
It is also known that cracking is even more pronounced in the case of a rigid foundation layer, namely one with a high modulus of rigidity.
In attempts to address these problems of road traffic-associated cracking, various measures of slowing down crack propagation have been proposed in the literature. All these measures aim at laying an intermediate layer between the top layer and the foundation layer. These measures include the following:
All these different solutions recommend installing an interface between the top layer and the foundation layer for the purpose of retarding the upward crack propagation into the top layer.
These solutions can only be used for remedial purposes, because they are based on the assumption that the roadbed already presents cracks.
Moreover, these known techniques concern semirigid roadbeds, namely those with a foundation layer made from a material treated with a hydraulic binder such as cement aggregate or slag aggregate.
In practice, these techniques give more or less satisfactory results.